guest ::
| Home > Publications database > Herstellung thermisch gespritzter Schichten mit optimierten Spannungseigenschaften |
| Home > Publications database > Herstellung thermisch gespritzter Schichten mit optimierten Spannungseigenschaften |
| Book/Dissertation / PhD Thesis | FZJ-2017-00966 |
2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-200-9
Please use a persistent id in citations: http://hdl.handle.net/2128/13659
Abstract: Gasturbines are widely used in stationary applications for power generation as well as in aero applications due to their high efficiency. The use of ceramic thermal barrier coatings (TBC) on the thermally high loaded turbine components allows either to increase the gas inlet temperature and therefore the efficiency of the system or to increase the lifetime of the system while maintaining the gas inlet temperature on it´s initial level. The ceramic thermal barrier coatings are typically deposited by atmospheric plasma spraying (APS) or electron beam – physical vapor deposition (EB-PVD). While the latter leads to microstructures with beneficial strain tolerance, the APS scores with it´s high deposition rates and the lower thermal conductivity due to the lamellar structure of these coatings perpendicular to the loading direction. Several factors occur during high temperature operation of the gasturbines that can increase the stiffness of the thermal barrier coating, influence the stresses within the thermal barrier coating, and finally may lead to delamination of the coating. The residual stress state of the as-sprayed coating can have an important effect on the behavior of the coating in operation. The in situ measurement of the sample´s curvature during the atmospheric plasma spraying process allows the evaluation of the residual stress evolution during the process and the determination of the residual stress depth profiles in the as-sprayed coatings. However, determination of the change of the stress depth profile in operation is not possible by this technique. The incremental hole-drilling method on the other hand is a widely used technique to determine residual stress depth profiles in the near surface region of components. By a comparison of the two measurement techniques for as-sprayed coatings the enhancement of the incremental hole-drilling method on coating systems is aspired. This finally enables the determination of the change of the residual stress states within the coating, the substrate and especially at the interface between the coating and the substrate by the thermo-mechanical loadings occuring in operation. A fundamental understanding of the interaction between the coating microstructure and the stress evolution is required to enable specific fabrication of coatings with well defined residual stress states. The investigation of these interrelations is aim of the present work. For this purpose, the different particle conditions in the plasma plume were evaluated in this work. Therefore, measurements of the temperature and velocity of single particles along the plasma plume were performed and combined with a simulation of the particle trajectories and the heat transfer between plasma and particles. Experimental studies of single splats deposited on mirror-polished substrates showed a high variety of splat morphologies that were classified into four basic splat-types to allow a systematic evaluation. The formation of these splat-types was linked to distinct particle and substrate conditions. The introduced particle melting degrees enabled the quantification of the relative fractions of the different particle conditions and, consequently, the relative fractions of the splat-types based on the particle temperature measurements. The formation of the coating and the connected properties was interpreted as the statistical distribution of the impact of single particles with various conditions. It was further shown that the properties of the deposited coating exhibit a distinct correlation to the distributions of the particle conditions. This is in particular the case for the coating porosity, which influences the Young´s modulus and the stress evolution. While the Young´s modulus of the coating is well defined by the porosity and the bonding between the single splats, the stress evolution exhibits more complex relations. The stress evolution in a single splat is dominated by the occurrence of various plastic effects (stress relaxation factors) depending on statistical factors, and therefore an exact prediction is hardly possible. An exemplary description was conducted based on existing analytical models and own finite element and analytical calculations. The layering of the single splats during the coating formation process was interpreted in the context of the statistic distribution of the different splat-types. However, it was not possible to completely describe the stress evolution by the influencing parameters used in this investigation. Since the impact frequency of the splats showed an effect on the stress evolution, the influence of the interface temperature between the single splats during deposition on the stress relaxation factors is supposed to play a key role. The outcome of this work can be used as starting point for the fabrication of atmospheric plasma sprayed coatings with well defined residual stress states. This offers the possibility to optimize the residual stress states in terms of the operation conditions as well as the fabrication of suitable samples for the enhancement of the incremental hole-drilling method on coating systems.
; 
|
The record appears in these collections: |
PDF
Fulltext by OpenAccess repository